Look on the back of your router for a url (numerical Web address) that is the format of 123.456.7.8 In your PC Internet browser put said url in address bar and connect to your router. Your user id should be on the first page. If you have forgotten your password contact your service provider to reset.
However your user ID and preset password should be on the sticker on the back of your router. If none of this works contact your service provider.

You need to check the ip address of your actiontec router, the default ip address of linksys router is 192.168.1.1, they should not be the same or else you will not able to connect the router to the internet.

Let me also give you the site on how to add the linksys router to another existing router.

The linksys has 4 LAN port and 1 "Internet" port on the back - we ONLY use the LAN ports NEVER the Internet port - because your existing router is connected to the Internet - your linksys is connected to the LAN

For me this is the best Linksys router for gaming, its WRT54G since its very easy to configure and you can also load some third [party software on it that can man boost this router for internet and gaming ...

have you tryed bridge mode on the modem
which device is doing the routing dhcp only one can do the dhcp
is the internet light lit on the router, the router is set for ppoe for dsl
try turning off dchp on the router let the modem handle

I have att yahoo dsl which I installed about a year ago. The process required bridging the ADSL Modem (in my case a speedstream 4100) and accepting dynamic IP addresses and configuring PPPoE on the WAN side of my Linksys router. The ATT support person walked me through the entire process over the phone even though I was comfortable with making the Linksys configuration changes as I had to do the same in China without technical support. The modem configuration was no more difficult than the router configuration as it was performed from a web browser too. The biggest warning was that to reverse the modem change (if desired) required use of a reset button because the web interface was disabled as a result of the configuration change. My advice to you especially if you are willing to attach the Linksys router to the telco modem is to contact the ATT highspeed DSL support line and see if they will walk you through it like they walked me through it. Reviewing your Linksys User's Guide and understanding the configuration of that device would probably be appreciated by the ATT Support person as well as assuring you can access the Linksys router to configure it during the call.

There is a small reset button on the rare side of Linksys router. You need to press that button.
Remember if you reset the router, it will come back to factory default settings and you will need to reconfigure it.
See How to reset Linksys router

sometimes this router are afected by the lightnings, verify that the internet connection is working well by plugin the internet(ethernet) cable directly to your computer, if it gets connected to the internet we are sure the probleme is isolated to the router.
The easiest way to check if the problem is the router WAN(internet) port would be connecting a known good router just for few minutes